1. Field of the Invention
The invention relates to a semiconductor laser having a double heterodyne structure. More particularly, the invention relates to a semiconductor laser which uses a semiconductor of gallium nitride type compound suitable for emission of blue light, which is capable of reducing operating voltage without reducing the light emitting efficiency.
2. Background Art
In the past, blue LED had a fault in putting it to practical use because it has lower luminance than a red LED or a green LED, but in recent years the luminance of the blue LED has increased and is in the spotlight now as a semiconductor of gallium nitride type compound has been in use, making it possible to obtain in p-type semiconductor layer of a low resistance containing Mg as a dopant.
The semiconductor of gallium nitride type compound described here is referred to a semiconductor in which a compound of Ga of group III element and N of group V element or part of Ga of group III element is substituted by other group III element such as Al and In and/or a semiconductor in which part of N of group V element is substituted by other group V element such as P and As.
In a conventional manufacturing method, gallium nitride type LEDs were manufactured in such processes as described below, and a perspective view of LED which uses a semiconductor of completed gallium nitride type compound is shown in FIG. 11.
First, by the organometallic compound vapor phase growth method (hereinafter referred to as MOCVD method), the carrier gas H2 together with trimethyl gallium which is an organometallic compound gas (hereinafter referred to as TMG), ammonia (NH3) and SiH4 and the like are supplied as a dopant to a substrate consisting, for example, of sapphire (single crystal Al2O3) at low temperature of 400° to 700° C., approximately 0.10 to 0.2 μm of low temperature buffer layer 2 consisting of n-type GaN layer is formed, and then the same gas is supplied at high temperature of 700° to 1200° C., and approximately 2 to 5 μm of high temperature buffer layer 3 consisting of n-type GaN of the same composition is formed. The low temperature buffer layer 2 is formed by polycrystalline layer to ease the strain caused by mismatching of the lattice between a substrate 1 and the single crystal layer of semiconductor and then turned into a single crystal by being subjected to 700° to 1200° C., in order to match the lattice by laminating the single crystal of the high temperature buffer layer 3 on that single crystal.
Further, material gas of trimethyl aluminium (hereinafter referred to as TMA) is added to the foregoing gas, a film of an n-type AlkGa1−kN (0<k<1) layer containing S of n-type dopant is formed, so that approximately 0.1 to 0.3 μm of an n-type clad layer 4 is formed to form a double heterodyne junction.
Then, instead of TMA which is the foregoing material gas, trimethyl indium (hereinafter referred to as TMI) is introduced to form approximately 0.05 to 0.1 μm of an active layer 5 consisting, for example, of InyGa1−yN (0<y<1), a material whose band gap is smaller than that of the clad layer.
Further, impurity material gas is substituted by SiH4 using the same material gas used for forming the n-type clad layer 4, Mg as a p-type impurity of biscyclopentadiene magnesium (hereinafter referred to as Cp2Mg) or dimethyl zinc (hereinafter referred to as DMZn) for An is added and introduced into a reaction tube, causing a p-type AlkGa1−kN layer which is a p-type clad layer 6 to be grown in vapor phase. By this process, a double hetero junction is formed by the n-type clad layer 4, active layer 5, and p-type clad layer 6.
Next, in order to form a contact layer (cap layer) 7, Cp2Mg or DMZn is supplied as the impurity material gas using the same gas as the foregoing buffer layer 23 to form 0.3 to 2 μm of the p-type GaN layer.
Afterward, a protective film such as SiO2 and Si3N4 is provided all over the surface of the grown layer of a semiconductor layer, aniline or electron is irradiated at 400° to 800° C. for approximately 20 to 60 minutes to activate the p-type clad layer 6 and the contact layer (cap layer) 7, after the protective film is removed, resist is applied and patterning is provided to form an electrode on the n-side, part or respective grown semiconductor layers is removed by dry etching so as to expose the buffer layer 3 or the n-type clad layer 4 which is the n-type GaN layer, an electrode 8 on the n-side and an electrode 9 on the p-side are formed by sputtering and the like, and AN LED chip is formed by dicing.
As a conventional semiconductor laser, one that uses a semiconductor of GaAs type compound is known, in which a resonator is formed by a double hetero junction structure with both sides of an active layer being held between clad layers consisting of a material having greater band gap energy and, smaller refractive index than the material of such active layer, so that it is possible to obtain the light oscillated in such resonator. Shown in FIG. 12 is an example of a semiconductor laser which uses a semiconductor of GaAs type compound having a refractive index wave guide structure provided with a difference of refractive index by an absorption layer in order to confine the light in the stripe portion of the active layer.
In FIG. 12, the numeral 121 represents a semiconductor substrate consisting, for example, of an n-type GaAs, on which are laminated in order a lower clad layer 124 consisting, for example, of an n-type AlαGa1-αAs (0.35≦α≦0.75), an active layer 125 consisting, for example, of AlβGa1-β-As (0<β≦0.3) of non-doping type or an n-type or a p-type, a first upper clad layer 126a consisting of a p-type AlαGa1-αAs, a current laminating 120 consisting of an n-type GaAs, a second upper clad layer 126b consisting of a p-type AlαGa1-αAs, and a contact layer (cap layer) 127 consisting of a p-type GaAs, and the p-side electrode 128 and the n-side electrode 129 are respectively provided on the upper surface and the lower surface in order to form a chip of a semiconductor laser. In this structure, the current limiting layer 120 consisting of the n-type GaAs restricts the injection current to the stripe-like active area of width W, by absorbing the light generated by the active layer, a difference of refractive index is provided in the inside and the outside of the stripe. Therefore, the semiconductor laser of the present invention is used as a semiconductor laser of a red or infrared ray refractive index wave guide structure wherein the light is confined in transverse direction and the wave of stripe-like active area of width W is directed stably.
In the semiconductor laser of such structure, a blue light radiating semiconductor laser using a semiconductor of gallium nitride type compound is also requested.
In a conventional semiconductor of gallium nitride type compound, the light emitting efficiency of the light emitting element of double hetero junction is high but the operating voltage thereof is high. If a material of small band gap energy, that is, a material of small Al composition rate k of AlkGa1−kN is used for the n-type clad layer and the p-type clad layer in order to lower the operating voltage, the operating voltage is lowered but the electron outflow from the active layer to the p-type clad layer increases, while the light emitting efficiency is lowered.
In the case where a semiconductor laser is to be composed by using a semiconductor of gallium nitride type compound, by providing a structure wherein an active layer is interposed between both sides by a clad layer consisting of a material having greater band gap energy and smaller refractive index than such active layer so as to confine the light in the active layer for oscillation, it can be considered to use InyGa1−yN (1<y<1, where y=0.1 for example) as the active layer and AlkGa1−kN (0<k<1, where k=0.2 for example) as the clad layer of both sides.
In a semiconductor laser which uses a conventional semiconductor of arsenic gallium type compound, specific resistance of AlαGa1-αAs as the clad layer is approximately 100 Ω·cm and there occurs no problem of increased operating voltage or heat generation even if such clad layer is used as one requiring approximately 1 to 2 μm, but if a semiconductor of gallium nitride type compound is used, the specific resistance of AlkGa1−kN (k=0.2 for example) is approximately 1000 Ω·cm when the carrier density of 1017 cm−3, which is approximately 8 times as compared with the specific resistance of GaN of the same carrier density, thereby increasing the operating voltage as well as the power consumption in addition to the problem of heat generation.
Further, in a light emitting element of a semiconductor which uses a conventional semiconductor of gallium nitride type compound wherein the GaN layer is used as the contact layer 7 in which the p-side electrode is to be made, due to such reasons that the GaN layer is affected by variations of the surface level and that there is a large energy gap between the metallic conduction band such as the alloy of Au or Au and Zn used as electrode and the valence band of GaN, and the contact resistance between the electrode metal and the cap layer does not stabilize as a result, so that the contact resistance becomes large and the operating voltage also rises. These problems results from a basic problem that it is not possible to increase the carrier density of the p-type layer, and further, in a type of semiconductor laser with the current injection area being restricted to stripe-like shape in which the contact area of the electrode is formed into a stripe-like shape, and the problem becomes more conspicuous.
Furthermore, as described above, the light emitting element of a semiconductor which uses a conventional semiconductor of gallium nitride type compound is composed by laminating on a sapphire substrate a semiconductor layer of gallium nitride type compound by means of a low temperature buffer layer consisting of GaN and a high temperature buffer layer, the lattice constant 4.758 Å of the sapphire substrate is largely different from the lattice constant 3.189 Å of GaN, the interatomic bonding strength of GaN is strong although it is weaker than that of AlGaN group, so that a crystal defect or transition is likely to occur due to temperature shock. In case a crystal defect or transistion occurs in the low temperature buffer layer, the crystal defect or transition progresses to the semiconductor layer formed thereon, thereby deteriorating the light emitting characteristic and reducing the life.
In addition, in the light emitting element of a semiconductor which uses a conventional semiconductor of gallium nitride type compound, as described above, electric current flows between the p-side electrode 8 provided on the contact layer 7 and the n-side electrode 9 provided on the high temperature buffer layer 3 which is an n-type layer due to the voltage applied therebetween, and the electric current flowing to the n-side electrode 9 has high carrier density of the buffer layers 2 and 3, so that the electric current flows throughout the buffer layers 3 and 2. On the other hand, because the buffer layer, the low temperature buffer layer 2 in particular is formed on a substrate consisting, for example, of sapphire that has a different lattice constant from that of a semiconductor of gallium nitride type compound, crystal defects or transition are likely to occur. When electric current flows into the buffer layer where crystal defects or transition take place, crystal defects or transition increase further due to the heat generated by the electric current, such crystal defects or transition progress to the semiconductor which contributes to the emission of light, thereby lowering the light emitting characteristic, reliability or the life.